This invention relates to a device comprising a thin film composed of a piezoelectric lead zirconium titanate compound and applied to a substrate formed of silicon or gallium arsenide. More particularly, this invention relates to such device comprising an intermediate film formed of non-piezoelectric lead-free zirconium titanate material to improve adhesion of the piezoelectric film to the substrate.
Piezoelectric materials are utilized in many types of electronic devices. For example, a surface acoustic wave filter, commonly referred to as a SAW filter, typically comprises spaced transducers attached to the surface of a piezoelectric film. An oscillating electrical signal applied to a first transducer creates acoustic waves that transverse the piezoelectric surface and, depending upon frequency and spacing, stimulates a second transducer to produce an output electrical signal. Such filters are effective to select signals having frequencies within a relatively narrow passband from a broad spectrum.
A preferred piezoelectric material is composed of lead zirconium titanate compound. As used herein, lead zirconium titanate refers to an oxide containing, as metallic constituents, predominantly lead, zirconium and titanium and optionally containing a minor amount of lanthanum, niobium, antimony, tungsten, tantalum, barium, strontium, neodymium, tin or other metallic agent in substitution for a portion of the lead to enhance particular electrical properties thereof. A thin film of the compound is sputtered onto a substrate, after which it is necessary to anneal the film at a temperature between about 500.degree. C. and 750.degree. C. to form a fine crystalline microstructure that brings about the desired piezoelectric properties. It has been proposed to deposit the piezoelectric film onto a silicon substrate of the type used in forming an integrated circuit die to permit the film to be combined with other electrical features. However, the compound contains lead oxide that reacts with silicon at the relatively high temperatures required to anneal the lead zirconium titanate and produces a glass phase at the interface that tends to crack, even during cooling following anneal, resulting in spalling of the piezoelectric film. This problem is exasperated by stresses created by differences in the coefficients of thermal expansion between the lead zirconium titanate compound and the silicon. Lead zirconium titanate similarly exhibits poor adhesion when applied to a substrate formed of gallium arsenide, an alternate material for integrated circuit die. Moreover, diffusion of lead into the gallium arsenide significantly affects the electrical properties of the substrate.
It has been proposed to apply platinum and titanium layers to separate the lead zirconium titanate and the substrate. The lead zirconium titanate is deposited onto the platinum layer to prevent interaction with the silicon, whereas the titanium forms a protective barrier to inhibit interaction between the platinum and the silicon that would otherwise occur at the lead zirconium titanate annealing temperature and lead to crack formation. These dual layers not only contribute significantly to the cost, but also, being formed of conductive metal, may affect electrical features in the substrate. Moreover, at the annealing temperatures, interaction between the titanium and the platinum layers tends to produce a rough surface in the piezoelectric layer that causes significant acoustic losses. Thus, there remains a need for a piezoelectric device that permits the preferred lead zirconium titanate compound to be applied to a substrate formed of silicon or gallium arsenide without requiring a metallic barrier and without reducing the acoustic properties of the piezoelectric layer.